Kaon (K±)

Strange meson whose puzzling production and decay led to the concept of strangeness and quark flavor.

Core Principles & Analysis

• First seen in 1947 as V-shaped cloud-chamber tracks by Rochester and Butler.
• Carries a strange quark; its odd behavior led Gell-Mann and Nishijima to the strangeness quantum number.
• The neutral kaon system revealed CP violation in 1964, earning Cronin and Fitch the Nobel Prize.

Specifications & Metrics

• SPIN: 0
• SYMBOL: K±
• CHARGE: ±1 e
• MASS: 493.677 ± 0.015 MeV/c²
• MEAN LIFE: 1.238 × 10⁻⁸ ± 2 × 10⁻¹¹ s
• PDG ID: S010
• PDG EDITION: PDG 2026

Principal Decay Modes

• K+ → e+ nu_e — 0.001582 ± 0.000007%
• K+ → mu+ nu_mu — 63.56 ± 0.11%
• K+ → pi0 e+ nu_e — 5.07 ± 0.04%
• K+ → pi0 mu+ nu_mu — 3.352 ± 0.034%
• K+ → pi0 pi0 e+ nu_e — 0.00255 ± 0.00004%

Branching fractions: Particle Data Group, 2026 edition (CC BY 4.0).

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